It is known for a nozzle in hot runner injection molding apparatuses to include a valve pin gating mechanism at each gate into each mold cavity. The valve pin is typically moved in a melt channel of the nozzle towards or away from the gate, to control the flow of melt into the melt cavity. In order to provide a good seal at the gate, both the tip portion of the valve pin and the corresponding sealing surface on the gate must typically be machined to very close tolerances
Due to a variety of reasons, however, the tip of the valve pin may be misaligned with the gate as it enters the gate. For example, the nozzle in which the valve pin moves may be misaligned with the gate. Also, thermal expansion and contraction of the components of the injection molding apparatus, which takes place repeatedly during an injection molding campaign can cause components to shift, ultimately resulting in misalignment of the nozzle and valve pin with the gate. Non-homogeneity in the melt itself can cause the melt to exert uneven fluid pressure on the valve pin body, which can push the sealing end of the valve pin out of alignment with the gate.
When a misaligned valve pin is moved to close a gate, the valve pin collides with the gate and can cause scoring of the sealing surfaces on the valve pin and/or the gate. This can ultimately result in poor quality parts with blemishes around the gate, and can cause other problems with the molding operation. Furthermore, a damaged valve pin or gate can be expensive and time consuming to replace. The damage may happen immediately, or alternatively it may happen gradually, over many cycles of opening and closing the valve pin.
Solutions that have been proposed for this problem, have typically included a guide means positioned towards the bottom of the nozzle melt channel to capture and align the free end of the valve pin. Because melt is required to flow past the alignment means/valve pin interface when the valve pin is in the open position, a plurality of circumferentially spaced slots are typically provided in either the valve pin or the alignment means. These slots create the potential for weld lines to appear in the molded product, as a result of the melt flow in the nozzle melt channel separating to pass around the guide means, and subsequently reuniting downstream from the guide means. Furthermore, the presence of such guide means in the nozzle melt channel typically renders more difficult a cleanout of the nozzle melt channel, hampering for example the changeover of a machine to run a new melt.
Other solutions have provided an offset nozzle melt channel which has a main portion that is offset from the center of the nozzle, and a lowermost portion that is aligned with the gate. The valve pin passes through the nozzle body and extends only into the lowermost portion of the nozzle melt channel. In this way, the valve pin is captured along a substantial portion of its length, which makes it less susceptible to misalignment. However, because a substantial portion of the nozzle melt channel is offset from the center of the nozzle, the heat distributed to the melt flowing therethrough is uneven, which can cause difficulties in controlling melt temperature. Reference is made to U.S. Pat. No. 5,834,041 (Sekine et al) and U.S. Pat. No. 5,895,669 (Seres, Jr et al), which disclose embodiments of this genre of proposed solution.
Other problems also exist, which originate from the manufacture of the nozzles themselves instead from the properties of the melt flow. Manufacturing errors may exist in the nozzles, which can introduce a misalignment between the valve pin and the gate that is ‘built-in’. The guide means that are described above, which are built into the nozzle itself, do nothing to correct this particular cause of misalignment.
Another issue relating to the valve pin has to do with the heat transfer characteristics of the nozzle and the mold plate. Typically, a space exists between the downstream end of a nozzle and the gate of the mold plate. The space is defined in part by the mold plate and in part by components of the nozzle. The space typically fills with melt at the beginning of an injection molding campaign. In some configurations of nozzle and mold plate, the melt in the space solidifies as a result of the combined effects of cooling from the mold plate, and insufficient heat transfer from the nozzle components. In some circumstances the solidified melt in the space can extend into the path taken by the valve pin towards the gate in the mold plate. Thus, in those circumstances, during movement of the valve pin towards the gate, the valve pin contacts the solidified melt in the space. The contact with the solidified melt can push the leading edge of the valve pin out of alignment with the gate. Furthermore, the solidified melt can cause wear on the leading edge of the valve pin, particularly if the melt is a glass-filled plastic or is some other abrasive material. Thus, it is possible for the valve pin to incur wear on its leading edge and sealing surfaces even when it is in perfect alignment, depending on the thermal characteristics of the molding operation and the material being injected.
Thus, a need exists for an injection molding apparatus that provides improved guiding of the valve pin towards the gate.